The use of radioactively marked isotopes in biology laboratories has expanded to a very great degree in recent times, primarily because of the growth of molecular biology and its increasing industrial applications.
A typical experiment in molecular biology consists in causing nucleic acids (DNA-RNA) to migrate on a gel by means of electrophoresis and to combine them with radioactively marked fragments of specific nucleic acids to form hybrids. During the electrophoresis the molecules migrate at a rate that is inversely proportional to their mass. The location on the gel of the radioactive fragments combined with the nucleic acids that have undergone electrophoresis makes it possible to identify the mass of the latter, which identification provides the biologist with a considerable amount of information.
The problem of identifying the position of radioactive isotopes is also encountered in biology, especially in the following operations:
analysis of "blots" or DNA sequences distributed in spots or specific areas over the gel, where the requirement is to analyze the spatial position of the blots, PA1 DNA sequencing, PA1 analysis of recombinatory phage ranges, PA1 one-dimensional and two-dimensional electrophoresis of proteins.
The biological products used in these techniques are usually marked with radioactive isotopes (.sup.32 P, .sup.35 S, .sup.3 H) which sporadically emit an electron having a continuous energy spectrum with a maximum in the vicinity of Eo/2 where Eo is the maximal energy of the electron (Eo=1.7 Mev, 167 Kev, 18 Kev for .sup.32 P, .sup.35 S, .sup.3 H, respectively).
Given the importance of these techniques in modern biology, it is essential to optimize the spatial detection of the radiation emitters. This is made all the more difficult by the fact that the spatial resolution required for such detection (less than 1 mm) has to be achieved for at least one dimension over large surfaces (in the order of 30 cm.times.20 cm), as the experimental results for DNA sequencing by electrophoresis are in the form of strips distributed over surface areas of this magnitude; each strip identifies the presence of a specific nucleotide and has to be discriminated spatially from the next strip, in particular with regard to its transverse dimension.
At present detection relies on autoradiographic film. Although this method offers good spatial resolution, dependent almost exclusively on the grain size of the photographic emulsion used, it has a number of disadvantages including low sensitivity, the fact that it is difficult to measure radiation intensity because of saturation phenomena and the need for subsequent analysis by visual observation, of necessity entailing a subjective interpretation.
To remedy some of the aforementioned disadvantages work has been done on designing high resolution autofluoroscope type instruments. Instruments of this kind have been described, for example, in the journal "Nuclear Instruments & Methods in Physics Research" Vol. 216, 1983 - Nov. No. 3 Amsterdam, Netherlands, in an article entitled "Scintillator - Fiber charged - particle track - imaging detector" by W. R. Binns, M. H. Israel and J. Klarmann.
Although they make it possible to achieve an acceptable degree of resolution by virtue of their use of an array of scintillation optical fibers, devices of this kind cannot achieve counting rates better than ten impacts per second because their signal processing system uses a CID type image intensifying system comparable to the charge transfer device in a video camera.